A hybrid electric vehicle powertrain typically has two power sources, one of which may be an internal combustion engine and the other of which is an electric motor. A high voltage battery, the motor, and a generator are electrically coupled using a high voltage bus. Each power source is drivably connected to vehicle traction wheels through transmission gearing. In a so-called power-split hybrid electric vehicle (HEV) powertrain configuration, the engine and the generator are drivably coupled using a gear set, usually a planetary gear set, thereby establishing parallel torque flow paths to the transmission wheels. The motor may act with the engine to complement mechanical torque delivery to the traction wheels, or it may act as a generator to charge the battery in a power regenerative mode. Another configuration for a hybrid electric vehicle powertrain is a series configuration of an engine, a generator, and a motor wherein the battery acts as an energy storage system for the generator and the motor.
The two power sources of a power-split hybrid electric vehicle (HEV) work together seamlessly to meet a driver's torque demand without exceeding powertrain system limits, including battery state-of-charge limits, while optimizing the total powertrain system efficiency and performance. A vehicle system controller will interpret a driver's demand for torque and then determine when and how much torque each power source needs to provide to achieve target fuel economy, emissions quality, and drivability standards. In a series HEV configuration, on the other hand, an engine drives a generator, which in turn drives a motor that is mechanically coupled to the vehicle traction wheels. Again, a battery, which acts as an energy storage device, is capable of providing motive power to the motor or accepting a charge from the generator depending upon the state-of-charge of the battery.
Another general category of a hybrid electric vehicle powertrain is referred to as a plug-in hybrid electric vehicle (PHEV) powertrain that includes a controller for determining power delivery from each of the two power sources using a battery charge depletion strategy. Unlike the control strategy for a parallel hybrid electric vehicle configuration and a series configuration, the battery is charged using an electric utility power grid following vehicle operation in which the electric motor may be the sole power source until the state-of-charge reaches a low threshold.
A first type of plug-in hybrid electric vehicle (PHEV) powertrain uses the engine to drive a generator, which in turn charges the battery that powers the motor after a low battery state-of-charge is reached. In another type of PHEV, only an electric motor drive is available whereby the vehicle drive range would be determined only by the state-of-charge of the battery.
In such PHEV powertrains, the power grid will charge the battery when the vehicle is not being used; e.g., when the vehicle is parked. The driver may connect the vehicle battery to the power grid following partial depletion of the battery charge by using a specially designed extension cable to plug one end into a wall socket in a residence or garage, the other end being connected to a terminal on the vehicle. When the vehicle owner wishes to use the vehicle, the cable would be removed to disconnect the vehicle and the wall socket. This is an inconvenience that the owner of a conventional vehicle does not have. If the owner, through oversight, fails to plug-in the vehicle following vehicle operation during which the battery state-of-charge becomes partially depleted, the benefit of the plug-in hybrid would be eliminated; for example, when the vehicle is to be used after the vehicle has been parked overnight.